With the exception of light-based sensors that change their light interaction properties, all sensors require some power in order to operate and provide a signal to a remote source. Light based systems are readily blocked by typical obstructions such as buildings, trees, and vegetation. Some wireless systems require the use of on-board circuitry that temporarily charges up a battery or capacitor in the presence of an externally applied RF radiation, then use this electrical energy to re-transmit signal. This method is bulky, expensive, and can only transmit data at short distances. The need for a powered sensor/transmitter severely limits the deployment of such sensors in large scale such as over large geographic regions or as part of the civil infrastructure.
Thus, it is desirable to provide a way to wirelessly transmit information about the local environment without the need for local power.